This invention relates to methods for the fabrication of a fabric-reinforced resin composite structure. More particularly, the invention is directed to the fabrication of a composite structure having a precisely determined, aerodynamically smooth outer surface (i.e., free of wrinkles, indentations and other irregularities). The invention is especially suited for fabrication of tubular or shell-type radomes and other structures.
In recent years, the use of high strength-to-weight ratio fabric-reinforced composites has grown dramatically, particularly in the aerospace industry where components that are constructed totally or partially of such materials can provide substantial weight savings without sacrificing strength or structural integrity. The ability to produce relatively large, shell-like structures from such composites has also increased their usage in the aerospace field. In one method of fabricating composite structures, plies or sheets that are formed from woven fibers and impregnated with partially cured resin ("prepreg") are laid up on (or within) a mold.
To make a composite having a precisely dimensioned smooth inner surface, the prepregs are laid over and cured on a male mold. To make a composite having a precisely dimensioned, smooth outer surface, the prepregs are laid up and cured in a female mold. In these conventional fabrication processes, a flexible vacuum bag is placed over the material laid up on the mold. The bag is then sealed and evacuated and the assemblage of the mold, laid up material and vacuum bag placed in an oven or autoclave and cured at an appropriate temperature and pressure. During this process, the resin contained in the preimpregnated reinforcing material or applied to the unimpregnated reinforcing material during layup flows to consolidate the various layers or plies into a high-strength, rigid, unitary structure. To ensure maximum strength and consolidation of the structure, the curing process must be closely controlled and monitored so that relatively uniform heat and pressure are applied to all portions of the structure being fabricated. Without such control, the strength and quality of the composite structure will be less than optimal because of uneven resin flow, voids, uneven plies or other defects, and other factors. In many instances, these imperfections will mean that the composite structure cannot be used and must be scrapped.
When these conventional techniques are employed, only the surface of the composite that is adjacent the surface of the mold will be relatively smooth. Sometimes it is possible to remedy this situation after cure by applying a filler material to local indentations and by sanding or otherwise machining the surface. The additional processing steps, however, increase fabrication costs. The composite must also be specifically designed to accommodate such operations because filling and sanding, or machining, can adversely affect the structural integrity of the composite. Of even greater importance, such operations often are impractical or impossible for aircraft parts that must be aerodynamically smooth (e.g., having a maximum deviation from lofted surface on the order of .+-.0.032 in. (.+-.0.081 cm) and a maximum depth-to-length ratio on the order of 0.003). Since a radome serves as a "window" for electromagnetic energy, its thickness, composition, and smoothness of the surfaces must also be closely controlled to reduce diffraction effects. The method of the present invention virtually eliminates machining or post-molding preparation of the surface finish of composites.
U.S. Pat. No. 4,067,950 discloses a prior art compression molding process for making radomes wherein a stocking-shaped webbing is inserted in a female mole and is filled with resin-impregnated fiber pellets. A male mold is then urged into the female mold as heat is applied, melting the resin and pushing it through the webbing. When curing is complete, the mold is cooled and a radome having smooth inner and outer surfaces is removed. Although prepreg material can be compression molded, the axial profile of most radomes prevents the use of prepreg material in the process disclosed in U.S. Pat. No. 4,067,950. Specifically, except in limited cases, the prepreg material cannot be placed in the female mold and compressed without forming wrinkles and other irregularities. Further, because of the relatively high temperatures and pressures involved, the tooling is typically both heavy and expensive.
U.S. Pat. No. 3,479,666 discloses a method for manufacturing a military helmet. Prepreg sheets are laid upon a slightly inflatable rubber male mold that is rigid enough to be self supporting. A female mold is placed over the male mold preform. Inflation of the male mold forces the prepreg against the surface of the female mold while curing occurs. When large structures are to be fabricated, the method has several drawbacks. First, it is difficult to produce a male mold that is flexible enough to respond to internal pressure, while simultaneously being rigid enough to resist deformation during the layup process. Second, because the female mold must withstand the pressure applied by the male mold, it is generally relatively heavy and costly.
In U.S. Pat. No. 4,379,013, a tubular spar of a composite helicopter rotor blade is fabricated by winding prepreg tape on a disposable mandrel covered with a pair of disposable pressure bags formed from folded sheets of plastic. The assembly is placed in a female mold that is contoured to define the outer shape of the blade. The bags are then pressurized, forcing the prepreg material against the female mold cavity while heat is applied to cure the resin. As with the methods discussed previously, when large parts such as radomes are to be fabricated, the female mold required is generally relatively heavy. Fabrication costs include the disposable tools consumed. This method of fabricating composite structure is further incapable of producing a wrinkle-free inner surface.
In many instances, prior art techniques using a female mold to obtain the desired smooth outer surface finish are of limited utility. For example, a one-piece female mold is not practical for fabricating a radome that is 60 to 70 inches long and only 18 inches in diameter at its widest point, since it is virtually impossible to place the prepreg on the inner surface of the mold without wrinkles or other defects. Further, once cured, it is almost impossible to get the radome out of the mold without destroying the mold. Thus, in the prior art, radomes of axial profiles that do not permit use of a unitary female mold have been formed in sections and bonded together to complete the radome or have been formed on a male mold with the resulting loss of precise control of the outer surface.